The present invention is directed to bioreactors, and more particularly to a biological method and apparatus for removing pollutants, such as volatile organic compounds, odors, and biodegradable aerosol/particulates from air emissions and convert them into carbon dioxide and water.
Existing gas-phase biological reactors use microbial metabolic reactions to treat contaminated air. Biological treatment is effective and economical for low concentrations of contaminants in large quantities of air. The contaminants are sorbed from a gas to the water/biological fixed film or suspended growth, where microbial attack occurs. Through oxidative, and occasionally reductive reactions, the contaminants are converted to carbon dioxide, water vapor, and organic biomass. Although a number of different configurations exist, the major gas-phase biological reactors are known to be biofilter, biotrickling filters, and bioscrubber (see Reference 1). In the gas-phase biological reactors, an optimized balance of contaminated air, nutrients, oxygen, waters and microbial population is a key factor for better efficiency (see References 2, 3, 4, 5, and 6).
Biofilters are not, however, filtration units, as strictly defined. Instead, they are systems that combine the basic processes of absorption, adsorption, degradation, and desorption of gas phase contaminants. A biofilter uses microorganisms fixed to media (compost, peat, etc.). As the contaminated air passes through the bed, the contaminants sorb into the biofilm and are biodegraded. Biofilters usually incorporate some form of water addition to control moisture content and add nutrients. In general, the gas stream is humidified before entering the biofilter reactor. However, if humidification proves inadequate, direct irrigation of the bed may be needed. Over time, the media tend to compact and regular replacement is needed.
A biotrickling filter uses an inorganic packing material, such as diatomaceous earth, ceramic, glass beads, etc., on which biological fixed film grows. Water is sprayed on the top of the packed bed and contaminated air is fed count-currently or co-currently. Biotrickling filters are governed by many of the same phenomena as biofilters. However, since a biotrickling filter hosts a thriving microbiological population, excessive biomass growth and clogging are common problems.
In a bioscrubber, after initial contaminant absorption occurs, the contaminants are degraded in a separate aeration tank. Absorption of contaminants may be achieved in a packed column, spray tower, or a bubble column.
A gas-phase bioreactor is disclosed in U.S. Pat. No. 2,793,096, for using soil beds to treat odorous sewer gases. In the last ten years, more stringent environmental requirements have, however, renewed interest in gas phase biological reactors in the United States.
The following U.S. patents are directed to improving the efficiencies of existing gas-phase biological reactors.
Kahler, U.S. Pat. No. 4,999,302, rearranges a rotating biological contactor (RBC), a typical wastewater treatment unit, and feeds contaminated air into a series of chambers containing an RBC disc set. Contaminated air mainly short-circuits through the space between the RBC and the housing, and the air in the RBC disc set remains stagnant, which means that microorganisms in the unit could not be used fully.
Rupert, U.S. Pat. No. 5,413,936, provides rotation of a cylindrical vessel having a horizontal, longitudinal axis, filled with biofilter media. The purpose of rotation was to break up compacted media and to collapse any fissures. Although the rotation helps to reduce the compaction and destroy cracks, there is channeling in the media, which would be more apparent over time, and contaminated air would short-circuit.
Phipps, U.S. Pat. No. 5,714,379, generates biologically activated foam to treat contaminated air.
Hongo, U.S. Pat. No. 5,766,938, also modifies the RBC system, and uses a perforated high-density polyethylene disc with a water scooping device.
Seagle, U.S. Pat. No. 5,780,293, uses filtering media, such as activated carbon, or zeolite in a rotating drum, and passes contaminated air through a drum after it is scrubbed through a suspended growth solution. The scrubbed air escapes mainly through the space between the wall and drum and through cracks in the media.
The present invention improves the efficiency of gas-phase biological reactors by increasing the chances of meeting contaminants, oxygen, nutrients, and moisture (water) with microorganisms. Biofilters generally pass humidified, contaminated air through a thick layer of peat moss or soil. Over time, the media compacts so that contaminated air/oxygen moves through a shortcut passage or crack, and only the microorganisms present in the passage are exposed to contaminated air. This xe2x80x9cchanneling effectxe2x80x9d means that only a limited portion of the media is actually used. Although the media material is porous, the air does not pass through the pores of the media.
Byung Kim (see Reference 7) used random shape engineered media (a flat square of polyurethane punched in the center with a circular hole and cut in half) to observe that contaminated air passed through the space between the media. Little microorganism growth occurred inside of media pores, and only the surface of the media was actively used.
The channeling effect is also a problem for the biotrickling filter. Zhu and others (see References 8, 9, 10, and 11) observed dense biomass growth in his biotrickling filter and had to backwash regularly to avoid clogging. When water is sprayed and clogging starts, water forms a channel in the biotrickling filter and contaminated air follows the channeling passages. Again, in the biotrickling filter, the microorganisms outside of the channel passage have a limited chance to contact contaminated air, oxygen, nutrients, and moisture.
Zhu and others (see References 8, 12, and 13) found nitrate was a better nitrogen source, but nitrate is a limiting factor when a highly biodegradable substance is treated. It was also found that gas-phase contaminated air can directly contact microorganisms without passing through the liquid layer. In order to overcome the nitrate-limiting condition, a gas-phase nitrogen source was suggested. In the bioscubber, the chance of water drops meeting contaminated air is also limited. In order to increase the microorganisms"" chance to contact scrubbed contaminated-air, Yu and others (see References 14, 15, 16, 17) used a three-phase fluidized bed and found that suspended biomass and fixed film play different roles at different environmental conditions.
The principal object of the present invention is to provide a biofilter reactor which performs better than any other existing gas-phase biological reactor because it uses all areas of the foam media pores that are fully coated with biological fixed film. In contrast, known gas-phase biological reactors use limited surface areas of the biological fixed film due to the channeling effect.
One object of the present invention is to provide an improved biological treatment method and reactor for converting volatile organic compounds (VOCs), odors, and biodegradable aerosol/particulates in air emissions into carbon dioxide and water.
Another object of the present invention is to provide a biofilter reactor which includes all the advantages of a biofilter, a biotrickling filter, and a bioscubber. In particular, the reactor of the invention presents at least the following features:
a. Direct contact of contaminated air with microbial film and very thin water film in air emerged cycle.
b. Operational flexibility as a biotrickling filter. This can change nutrient content and concentrations. Water content in the media can be changed by varying rotational speed of the media.
c. By feeding contaminated air through the inlet in the bottom housing, the reactor can be operated as a bioscrubber.
Yet another object of the present invention is to provide a biofilter reactor which has no clogging problem. A pilot system worked more than a year without clogging (see FIG. 8). Rotating the media allows excessive biomass to slough off in the submerged phase. Therefore, regular media replacement is not necessary other than from wear and tear of the media about every two years.
An additional object of the present invention is to provide a biofilter reactor which occupies a much smaller area.
Yet another object of the present invention is to provide a biofilter reactor in which media thickness (depth) is less than two inches and an air gap of less than 0.5 inches means uniform distribution of contaminated air, oxygen, water, and nutrients to the microorganisms.
Still yet another object of the present invention is to provide a biofilter reactor which is cost competitive because of its simple design.
A further object of the present invention is to provide a biofilter reactor which can operate either in a biofilter mode or a bioscrubber mode.
In summary, the main object of the present invention is to provide a biofilter reactor for removing various pollutants from a fluid. The reactor performs better than known systems and has the advantages of a biofilter, a biotrickling filter and a bioscrubber.
In accordance with the invention, a biofilter reactor includes a housing, an axial pipe rotatably supported in the housing and including a plurality of perforations that open into the interior of the housing for collecting a treated fluid. The axial pipe includes an outlet in communication with the interior thereof for removing the treated fluid from the housing. A porous medium is disposed about the axial pipe and is rotatable therewith. The porous medium is made of a microbial foam.